Method of treating abrasive-laden drilling liquid



METHOD OF TREATING ABRASIVE-LADEN DRILLING LIQUID 2 Sheets-Shee 1 Filed Sept. 24 1963 April 2, 1968 oonwm ETAL 3,375,886

METHOD OF TREATING ABRASIVE-LADEN DRILLING LIQUID Filed Sept. 24, 1965 2 Sheets-Sheet j INVENTORS. fiaaserJ. 6000M mA/asm. uon/ JdSEPl/LPEAAREA 404 mscmua ATTOAIVE'V.

United States Patent 3,375,886 METHOD OF TREATING ABRASIVE-LADEN DRILLING LIQUID Robert J. Goodwin, Oakmont, Ernest A. Mori, Hampton Township, Allegheny County, Joseph L. Pekarek and Paul W. Schaub, Penn Hills Township, Allegheny County, and Robert E. Zinkham, Harrison Township, Allegheny County, Pa., assignors to Gulf Research 8; Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Sept. 24, 1963, Ser. No. 311,035 3 Claims. (Cl. 175-66) ABSTRACT OF THE DISCLOSURE A method of treating abrasive-laden liquid discharged from the well in a hydraulic jet drilling process by first separating cuttings larger than the abrasive particles from the liquid discharged from the well. The cuttings-free liquid from the first separation step is then treated to separate the abrasive particles from the liquid. The abrasives-free liquid is then treated to. separate fine particles smaller than the abrasive particles to provide a clean drilling liquid. The clean drilling liquid is mixed with abrasive particles for recirculationinto the well. The abrasive particles may be the particles separated from the drilling liquid. Before the drilling liquid is returned to the well it is passed through a cooler to remove heat developed by the large amount of energy transmitted to the drilling liquid. Treatment of a drilling liquid comprising two immiscible liquids is also described.

This invention relates to the drilling of boreholes in the earth and in particular concerns a method and apparatus for treating the drilling fluid employed with a hydraulic jet drill.

Conventional devices for drilling deep boreholes function by making physical contact of the cutting surfaces of a metal drill bit with the rock formation at the bottom of the hole to mechanically cut the rock away. Such drill bits as the well known fish-tail bit, drag bit, core bit, roller bit, cone bit, disk bit, etc., all operate to make hole by mechanically breaking up the rock at the bottom of the hole whence the cuttings are removed to the surface of the earth by means of a circulating fluid medium such as air, foam, or drilling mud. In mechanically breaking up the rock it is inevitable that a substantial amount of wear and breakage also occurs to the cutting elements of such a drill bit, so that eventually the bit wears out and can no longer make hole. The drill stem must then be withdrawn from the hole, the bit replaced, and the bit stem with the new hit reinserted in the borehole. In drilling hard formations in a deep hole the time spent replacing conventional drill bits may exceed the actual drilling time on bottom, and this results in a loss of efliciency and very substantially increases the expense of the drilling operation.

Hydraulic jets have heretofore been included with conventional drill bits, but these jets have been for the purpose of keeping the cutting edges of the drill bit, or the rock surface being cut, free from mud and chips produced by the bit and thereby increase the efliciency of the mechanical cutters on the bit. However, such ancillary jets as have been employed with conventional rock bits have no effective cutting action when operated with the pressures normally employed for fluid circulation in conventional mechanical drilling operations.

We have found that when hydraulic jets are operated at very high pressure so that extremely high velocities are attained by the emerging jet stream, the fluid jet is very effective in making hole in hard rock. By omitting substantially all mechanical cutters from the drill bit there. is obtained a bit that is substantially free of mechanical wear or breakdown. Accordingly when such a bit is used in the hole, it will remain effective to make hole for a much longer period of time than a bit that includes mechanical cutting elements. Also due to the fact that the jets tear up the rock into very small fragments, the cuttings are more easily removed by the circulating fluid than are the larger cuttings made by conventional mechanical drill bits. Furthermore, because of the mechanical simplicity of such an all-jet bit, such 'bits are very sturdy and are also relatively inexpensive, thus resulting in a further saving of drilling expense. In addition when such bits are operated in accordance with this invention they are found to make hole at a much faster rate than conventional mechanical bits.

We have found that when a plurality of hydraulic jet streams of extremely high velocity are rotated in a bore hole, very effective cutting action is obtained even in hard rock. In accordance with this invention, the velocity of said streams exceeds the critical minimum cutting velocity for the earth material being penetrated. It is to be noted that in contrast to prior-art drilling devices, there is substantially no physical contact of the tool em ployed to produce said hydraulic jets with thesurfaces of the earth formation being drilled or bored. Thus, the tool or bit of the present invention is substantially free of the Wear or breakdown occasioned by the physical contact of prior-art bit surfaces with the rock formation being drilled.

Although we have for convenience herein used the term bit to describe the drilling tool of the present invention, from the foregoing and the detailed description which follows it will be apparent to those skilled in the art that the process and apparatus of the present invention are entirely different from prior-art drill bits. A distinction is made between prior-art drill bits which are of the mechanical type wherein the rock-cutting action results from physical contact of the metal bit surfaces with the rock formation at the bottom of the hole, and the jet bits of this invention wherein the rock-cutting action results from the erosive action of a high-velocity jet stream issuing from a nozzle that does not contact the rock formation and which functions most advantageously when the nozzle is spaced a specified distance (called the standotf) from the rock being drilled.

In this invention a jet drill is employed to penetrate the earth to form a borehole. By a jet drill is meant one whose rock-outting action results substantially entirely from a plurality of hydraulic jet streams issuing from nozzles in the tool at a velocity that exceeds the critical minimum cutting velocity. The critical minimumcutting velocity is that fluid velocity below which substantially no cutting action of the jet takes place, whereas above this critical velocity the cut-ting action increases rapidly with increasing velocity. The critical minimum cutting velocity varies with the target material being drilled and also with the nature of the jetting fluid. The drilling fluid employed is pumped to the drill bit under pressure sufliciently high so that the jet stream emerging from the bit exceeds the critical minimum cutting velocity of the rock being drilled.

The cutting action of a high-velocity jet stream can be increased by the use of a fluid medium that contains entrained material, as for example sand. We have found as disclosed in copending application Ser. No. 311,088, filed of even date herewith and assigned to the same assignee as this application, that at velocities above the critical minimum cutting velocity the target removal rate of the jet varies with the sand concentration. We have found that with a particular type of entrained material, the cutting rate is larger for a particular range of concentrations of the entrained material and the cutting rate is less for concentrations above and below this range. One possible explanation of this is that if there is too little entrained material, the cutting rate is increased by the addition of more material until a concentration is reached that is so high that an appreciable number of entrained particles simply strike the top of a previous particle and fail to strike and cut the rock being attacked. Accordingly, it is desirable to maintain the drilling fluid in a condition so that it carries an optimum concentration of desirable entrained particles.

We have further found that upon passing through the drill bit at a velocity exceeding the critical minimum cutting velocity and returning to the surface of the earth, a substantial portion of the material entrained in the drilling fluid will be reduced in particle size. Thus the drilling operation produces fines at the expense of the useful sized entrained particles. The fines are inimical to the drilling operation because they increase the power required to develop a specified jet velocity and they get in the way of the useful particles to reduce cutting action of the drilling fluid in a manner similar to the effect of excessive particle concentration. The fines contribute nothing to the cutting action of the drilling fluid. Accordingly, it is desirable to remove the fines from the spent drilling fluid and add desirable sized particles to build up the concentration of the desirable sized particles before the drilling fluid is again recirculated to the jet bit.

It is an object of this invention to provide a method for jet drilling that maintains optimum conditions in the drilling system.

Another object of this invention is to provide a method for jet drilling that avoids excessive wear on the pumps and other equipment required.

A further object of this invention is to provide a method of jet drilling which in one embodiment avoids the use of abrasive particles in the drilling fluid.

These and other useful objects of this invention are attained by the method described in this specification with reference to the accompanying drawings forming a part thereof, and in which:

FIGURE 1 is a diagrammatic illustration of one form of jet drilling operation in which solid material is entrained in the drilling fluid and to which this invention is applicable;

FIGURE 1A is a diagrammatic representation of processing equipment that may be employed in this invention to reconstitute drilling fluid containing entrained solid particles; and

FIGURE 2 is a diagrammatic illustration of an embodiment of the invention in which an immiscible secondary liquid is entrained in the drilling fluid.

In accordance with this invention, material of a different density, either solid or liquid, then the primary constituent of the drilling fluid is entrained in the drilling fluid. When the jet fluid contains solid particles, the spent drilling fluid is screened to remove large rock chips, further screened to remove particles of useful size, and treated to remove the remaining fines. The jet drilling fluid is then reconstituted by adding new and used useful sized particles to restore their concentration to a desired value. The invention further provides a jet drilling fluid containing a secondary immiscible liquid whose particles are entrained in the drilling fluid in the form of droplets of substantial size. This type of jet drilling fluid is treated after use by screening out the cuttings, completely separating the two liquids, and then reinjecting one of the liquids as droplets of desired size in the other continuousphase liquid.

' Referring to FIGURE 1 there is diagrammatically shown a borehole penetrating rock formation 11 in the earth. The drill comprises a conventional drill stem 12 having at its upper end a swivel 13 and supported at least in part by a hoisting line 14in conventional manner.

The upper part of the borehole 10 is usually cased as indicated at 15. The drill stem is rotated in conventional manner by means of rotary table 16 powered by a prime mover 17. Drilling fluid under very high pressure is supplied to the drill 20 by pipe 18. As the drill makes hole, it is lowered by lowering of hoisting line 14 from the hoist. The rate at which drilling progresses is measured by a conventional drilling rate recorder 19 whose recording mechanism is connected to swivel 13 by line 24 in conventional manner.

The bottom of the drill stem carries a jet bit 20 indicated diagrammatically since its structure per se does not form a part of this invention. The jet bit 20 may, for example, be of the type described in our copending applications Ser. No. 311,034 and Ser. No. 311,088, filed of even date herewith and assigned to the same assignee as this application. Drilling fluid under high pressure is pumped down the drill stem as indicated by the arrows 21 and issues from the nozzles of bit 20 in the form of a very high velocity jet stream. The velocity of the jet stream issuing from the bit 20 exceeds the critical minimum cutting velocity so that rock cutting action takes place. The spent drilling fluid returns upward through the annular space around the drill stem as indicated b the arrows 22 and leaves the well through fluid return pipe 23.

The drilling action of the hydraulic jet stream issuing from the bit 20 may be augmented by having entrained in the drilling fluid a certain amount of solid material. We have found that drilling action is a maximum for a particular range of concentration of a particular entrained solid material. By way of example, as disclosed in the aforementioned application Ser. No. 311,088, when using entrained sand whose particle size is predominantly 20 to 40 mesh optimum penetration rate of the drill is obtained when the sand concentration is in the range between about 5 percent and about 15 percent by volume (using the A.P.I. procedure for measuring sand content of drilling fluid).

We have found that when drilling fluid containing entrained material strikes a target at a sufliciently high velocity to cut the target, the entrained material is broken up into smaller fragments. Thus when sand particles of a certain size are used in a jet drill, the sand particles are comminuted by the jet drilling process. Accordingly, after a jet drilling fluid has been in use for a comparatively short time, the entrained sand particles are no longer of optimum size and the drilling ability of the drilling fluid deteriorates. It is the purpose of this invention to provide a method and apparatus for maintaining a jet drilling fluid in optimum cutting condition by reconstituting the drilling fluid to contain material of optimum size and concentration.

In the case of the above-mentioned example wherein a sand concentration in the range between about 5 percent and about 15 percent is employed, the particle size of predominantly between 20 mesh and 40 mesh is determined by conventional sieving techniques. Particles larger than 20 mesh are considered impractical because of the resultant clogging of nozzles, pump valves, etc. We have found that the presence of entrained solid particles smaller than 40 mesh have substantially no beneficial effect on the drilling effect of the drilling fluid, whereas these smaller particles interfere with the drilling effect of the larger particles in a manner similar to the effect produced by excessive particle concentration. The fines also increase the density of the drilling fluid so that greater horsepower is required to obtain the same nozzle exit velocity. Furthermore, the presence of the very fine particles, particularly abrasive particles, substantially reduces the life of pumps and other equipment employed. Accordingly, in this invention the fines are removed from the spent drilling fluid as well as any large rock fragments that result from the drilling operation. The drilling '5 fluid is subsequently reconstituted to optimum condition prior to reinjection into the drill stemthrough pipe 18'.

Referring now to FIGURE 1A which shows a generally top view of the mud processing equipment, the spent drilling fluid returning from the well through pipe 23 is conducted to a double-screen shale shaker 25 that is driven by motor 26 in conventional manner. The shale shaker 25 has two vertically spaced superposed screens 27 and 28. The lower screen 28 has smaller openings than the upper screen 27 and is only partially shown in the figure where the upper screen is cut away for purposes of illustration. The spent drilling fluid from pipe 23 is discharged onto the upper screen 27 which retains the larger rock fragments, the latter being dumped into a trough 29. These rock fragments may be studied by the geologist but they are discarded as far as the drilling operation per se is concerned. The drilling fluid carrying smaller particles goes through screen 27 and falls on the lower screen 28 having smaller openings. Those particles that are retained on screen 28 are dumped into a trough 31 and carried away by pipe 32 to be reused as will be explained later. Drilling fluid that carries particles smaller than the openings in screen 28 goes through the screen 28 into a trough 36.

FIGURE 1A shows a top view of the shale shaker 25 located above one end of the mud tank 33. The tank 33 has a flow baflle 34 near the outlet end of the tank. The baffle 34 serves to separate stored drilling fluid from that being circulated, the latter being contained in the section 47. The drilling fluid that passes through screen 28 drops into the trough 36 and is put through a desanding operation. The fluid that passes through screen 28 contains fines produced both as a result of cutting of the rock being drilled and as a result of comminution of the originally entrained abrasive sand. Drilling fluid that falls into trough 36 passes via line 37 to a centrifugal pump 39 and into a battery of desanders 40 shown generally in elevation in FIGURE 1A. The desanders 40 may be conventional mud cyclones or special desanders known for removing sand and cuttings from drilling mud, as for example the Bowen Desander manufactured by S. R. Bowen Co., Santa Fe Springs, Calif. Three desanders 48 are shown, but any number may be used that is required to efficiently remove the fines from the fluid delivered by pump 39 to the input manifold 41. The desanders 40 extract the fine sand and cuttings which exit through the bottom discharge openings of the desanders. These fines are discarded. The desanded drilling fluid is delivered to manifold 42 and flows via pipe 43 to a cooling tower 44. From the bottom' of the cooling tower the drilling fluid is picked up by centrifugal pump 45 and passes via line 46 to trough 31. The desanded fluid washes the trough 31 and carries the useful sand particles from trough 31 via line 32 into the end 47 of the mud tank for reuse. Alternatively, the desanded drilling fluid from the cooling tower may flow through line 46, trough 31, and line 32 by gravity into tank 47. It is thus seen that the drilling fluid is continually being circulated through the desanders 40 and returned to the mud tank. The rate of circulation through pump 39 and the number of desanders 48 is adjusted with respect to the circulation rate through the well so that the drilling fluid in the output end 47 of the mud tank 33 has only a tolerable small concentration of fines.

, The solids caught on the lower screen 28 and which are dumped into trough 31 previously mentioned are washed by the desanded fluid from pump 45 and delivered by pipe 32 to the output end 47 of the mud tank. These solids are in the size range known to be desirable in the drilling fluid for the purpose of augmenting its cutting ability. In order to make up for solids lost through fines, sand 48 of desired size is added to the drilling fluid in 47 by means of a sand mixer 49 whose feed screw 51 is driven at controlled rate by variable-speed motor 50.

The drilling fluid in tank 47 may be kept well mixed by means of a circulating pump not shown. The concentration of sand in the drilling fluid in tank 47 may periodically be checked by taking a sample and. determining its sand concentration by standard A.P.I. procedure. The size of the sand particles in tank 47 is determined by the openings in the shale shaker screens 27 and 28. For the previously mentioned example, the openings of upper screen 27 are 20 mesh and those of lower screen28 are 40 mesh.

The drilling fluid in the output end 47 of the mud tank is thus reconstituted to carry the proper amount of solid abrasive particles of size that results in optimum drilling rate. The thus reconstituted drilling fluid is then pumped from tank 47 by means of one or more high-pressure pumps 52 to a manifold 53 and thence to the line 18 leading to the swivel 13 for circulation down the well to the jet bit 20. The line 18 is provided with a pressure gauge 55. The spent fluid returning from the well through pipe 23 returns again to the processing apparatus of FIG- URE 1A for retreatment.

This invention thus provides for removing from the spent drilling fluid those particles that are detrimental or inimical to the drilling operations. The openings in the upper screen 27 are of a size to retain particles that are larger than those desired, these usually being chips or large cuttings of the rock being drilled. The openings in the lower screen 28 are of a size to retain useful sized particles and pass the fines. Thus particles whose size is such as to be useful in the drilling fluid are delivered by the shale shaker 25 to trough 31 and these particles are reused by combining them with the drilling fluid in the output end 47 of the mud tank.

The pumps 52 deliver reconstituted drilling fluid to the drill at a sufliciently high pressure so that the pressure differential at the jets in the bit is sufficient to effect a nozzle exit velocity that exceeds the critical minimum cutting velocity. This is easily determinable by means of the drilling rate recorder 19. When drilling in any given rock, it is found that upon gradually increasing the pressure (as shown by gauge 55) the drilling rate will be practically nil for low pressures, and when the pressure reaches the critical minimum cutting pressure, the drilling rate will suddenly increase rapidly with further increase in pressure as the critical minimum cutting pressure is exceeded. We prefer to operate at a pressure as indicated by gauge 55 that exceeds 4000 p.s.i.g.

It is found that the critical minimum cutting pressure varies with the sand concentration. At a given pressure that exceeds the critical value, the drilling rate will be a maximum for some concentration value of entrained sand or other abrasive particles. Since the operator does not always know the nature of the rock in which the bit 20 is drilling, the optimum concentration is best found by simple experiment. The rate at which sand, for example, is added by means of sand mixer 49 is adjusted by adjusting the speed at which the sand-feed screw 51 is driven so as to maintain a maximum drilling rate as indicated on drilling rate recorder 19. By way of example, a concentration in the range between about 5 percent and about 15 percent of sand whose particle size is predominantly between 20 mesh and 40 mesh may be employed, but by means of this invention any desired concentration of any desired size may be maintained during the drilling operation.

Instead of using solid abrasive particles such as sand entrained in a jet drilling fluid, a secondary liquid of density substantially different from that of the main part of the drilling fluid and immiscible therewith may alternatively be employed as the additive to the drilling fluid to increase its drilling effect. Thus for example, about 10 percent by volume of carbon tetrachloride (density 1.6 g./ cc.) may be added to an aqueous drilling fluid to substantially increase the drilling rate of the mixture. The secondary liquid is dispersed in the drilling fluid as a loose emulsion or as droplets of substantial diameter, and should not be in the form of tight emulsion. The entrained droplets of secondary liquid will be substantially spherical due to surface tension. The entrained droplets preferably have a diameter substantially equal to the internal diameter of the nozzles employed in the jet bit 20, as for example /8 inch. These particles will maintain their size during transit down the drill stem with substantially no tendency to break up and very little tendency to coalesce, and thereby will emerge from the nozzles of the bit 20 as momentary discontinuities in the jet stream. Upon striking the rock at the bottom of the hole at high velocity, however, the droplets are disintegrated into much finer droplets and under some circumstances may become emulsified. The spent drilling fluid returns to the surface where it is reconstituted and reinjected into the drill stem for further drilling.

Referring to FIGURE 2 there is illustrated diagrammatically a drilling system employing a drilling fluid that comprises a primary liquid of continuous phase having dispersed therein as a discontinuous phase, droplets of a secondary liquid having a density substantially different from that of the primary fluid, and preferably of a higher density. In the figure the elements having the same reference numerals have the same function as similarly numbered elements of FIGURE 1 already described. The drilling fluid after passing through the jet bit 20 returns to the surface through the annular space as indicated by arrows 22 and is discharged through pipe 23. The spent fluid is conducted to a conventional motor-driven shale shaker or screening apparatus 61 which removes rock fragments brought to the surface by the drilling fluid. The screening apparatus 61 may have a series of screens, the last of which is sufficiently fine so that the apparatus retains substantially all solid particles. From the screening apparatus 61 the fluid, now substantially free of solids but comprising two liquids in substantially emulsified form, passes via pipe 63 to a means for separating the two liquids. Any effective means of separation may be employed, as for example a continuous centrifuge, electrical precipitator, or the like. Alternatively, small mesh screens or fabric that are wet by the primary liquid (continuous phase) but not wet by the secondary liquid may be employed to separate the liquids. Alternatively, an emulsion-breaking chemical may be added to the emulsified drilling fluid by means of a luhricator 64, after which the two fluids are allowed to settle by gravity in settling and cooling tank 65. In the settling tank 65 the heavier component will settle to the bottom of the tank at 66 and the lighter component to the top at 67. The two liquids are then separately drawn off through pipes 68 and 69 respectively. The lighter of the two liquids, namely 67, forms the larger primary part of the drilling fluid and this is pumped by means of pump 71 through a motor-driven slug injector 62. The heavier or secondary component liquid 66 is pumped by means of pump 70 into the interrupted flow line of the slug injector 62. The slug injector 62 injects substantially uniformly sized droplets of the secondary liquid 66 into the primary component stream 67, the droplets being of a size that effects maximum drilling rate. The slug injector 62 is preferably motor driven at a controlled rate so that the proper percentage of the secondary component liquid can be introduced. In its simplest form the slug injector 62 merely interrupts the stream of liquid from pump '70 as it enters the larger stream of liquid from pump 71 with the frequency of interruption related to the flow rate to effect droplets of the desired size. Alternatively the secondary liquid may be sprayed into a stream of the primary liquid (both liquids being under high pressure) so as to produce the desired sized droplets. It is preferred that pumps 74) and 71 develop the final jetting pressure to avoid putting the reconstituted drilling fluid in pipe 18 through an additional pump because a pump has a tendency to reduce the droplet size. However, an additional pump (not shown) may be employed in the line 18 to raise the pressure if necessary. From pipe 18 the reconstituted and pressurized drilling fluid is returned to the drill stem via swivel 13.

The use of an entrained secondary immiscible liquid of different density instead of entrained solid abrasive particles is advantageous in that wear of pumps is substantially eliminated, Furthermore, we have found that the use of a secondary liquid also avoids erosion of the nozzles in jet bit 20 whereas entrained solid particles effect some wear on the nozzles and other flow channels. In addition the secondary liquid is not destroyed, but is recovered and substantially completely reused thereby reduring the expense of the drilling operation.

By way of example, of liquids that may be employed but not by way of limitation, the primary liquid may be an oil such as crude oil, diesel oil, or the like, and the secondary liquid may be an aqueous solution of an inorganic salt in sufficiently high concentration to attain high density. Thus for example, the secondary liquid may be a 70 percent zinc chloride solution having a density of 1.96 g./cc., or a 40 percent calcium chloride solution having a density of 1.40 g./cc., these solutions being relatively inexpensive and affording a substantial density contrast to the primary liquid (oil). When an aqueous primary liquid is employed, other satisfactory secondary liquids are the liquid chlorinated diphenyls, as for example Aroclor 1248 manufactured by Monsanto Chemical Company, which has a density of 1.45 g./ cc.

While we prefer to employ droplets of the secondary liquid that are of substantially the same diameter as the nozzle openings in the jet drill being used, by means of this invention the drilling fluid can be made to carry a concentration and/or size of droplet that results in optimum drilling rate. By observing the drilling-rate recorder 19 and adjusting the rate of injection of secondary liquid as well as the size of the injected droplets, the operator can maintain maximum drilling rate for the rock being drilled.

Certain aspects of the invention herein disclosed are disclosed and claimed in copending applications Ser. No. 311,034 and Ser. No, 311,088, filed of even date herewith and assigned to the same assignee as this application.

What we claim as our invention is:

1. A method of earthbore drilling wherein a plurality of hydraulic jet streams issuing from nozzles in a drilling tool are employed substantially entirely to cut the rock being penetrated which comprises preparing a drilling fluid comprising a first liquid forming a continuous phase and having entrained therein droplets of a second liquid immiscible with the first liquid,

the density of said second liquid being different from that of said first liquid,

injecting the drilling fluid into the drill stem at a pressure to effect an exit velocity from the drill that exceeds the critical minimum cutting velocity for the material being drilled,

recovering the drilling fluid at the top of the borehole,

separating said second liquid from said recovered drillreconstituting the recovered drilling fluid by adding thereto droplets of said second liquid whose diameter is substantially equal to the diameter of the nozzles employed in said drill, and

reinjecting the reconstituted drilling fluid into the drill stem.

2. The method of claim 1 wherein said second liquid has a density higher than that of said first liquid.

3. In a method of drilling a borehole through hard formations wherein a plurality of hydraulic jet streams of a drilling liquid in which abrasive particles are suspended are discharged at high velocities from nozzles in a drilling tool against the bottom of the borehole for penetration of the formations drilled and the drilling liquid is circulated up and discharged from the borehole to carry cuttings from the well, the improvement comprising separating cuttings larger than the particles of abrasive from the drilling liquid discharged from the Well to form a cuttings-free drilling liquid and discarding the cuttings, separating abrasive particles from the cuttingsfree drilling liquid, separating fine particles smaller than the abrasive particles from the drilling liquid from which the abrasive particles have been separated to produce a clean drilling liquid and discarding the fine particles, passing the clean drilling liquid through a cooler, mixing the previously separated abrasive particles with the clean drilling liquid to reconstitute the drilling liquid, and recirculating the reconstituted drilling liquid down the Well to the drill bit.

References Cited UNITED STATES PATENTS 2,805,722 9/1957 Morgan et al. 252-855 3,040,822 6/1962 Graham et al 175-66 3,066,735 12/1962 Zingg 166-222 3,112,800 12/1963 Bobo 175-67 3,081,828 3/1963 Quick 175-67 3,140,747 7/1964 Mitacek 175-66 2,919,898 1/1960 Marwil et al. 175-66 2,941,783 6/1960 Stinson 175-66 JAMES A. LEPPINK, Primary Examiner. 

